Alkali-free glass substrate

ABSTRACT

An alkali-free glass substrate containing, by mass percent, 50-70% of SiO 2 , 10-25% of Al 2 O 3 , 5-20% of B 2 O 3 , 0-10% of MgO, 0-15% of CaO, 0-10% of BaO, 0-10% of SrO and 0-5% of ZnO, also containing SnO 2  and/or Sb 2 O 3  and having a B-OH value of at least 0.485/mm.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an alkali-free glass substrate, moreparticularly to an alkali-free glass substrate useful as a transparentglass substrate for displays and others.

2. Description of Related Art

Conventionally, an alkali-free glass has been used as a transparentglass for liquid crystal displays. The alkali-free glass particularlyfor display application is required to be free of bubbles that result indisplay defects, as well as having satisfactory heat resistance andchemical durability.

For such an alkali-free glass, various types of glasses have beenproposed. For example, Japanese Patent Kokai No. Hei 6-263473 andJapanese Patent Kohyo No. 2001-500098 disclose alkali-freealuminosilicate glasses.

If a bubble-free glass is to be obtained, a gas produced during a batchdecomposition must be expelled from a glass melt by a fining gas. Inaddition, the remaining minute bubbles must be removed duringhomogenization melting by the reproduced fining gas which renders themlarger in sizes and thereby causes them to float in the melt.

For the alkali-free glass, particularly for use as a glass substrate forliquid crystal displays, a glass melt has a high viscosity and itsmelting is carried out at higher temperatures compared toalkali-containing glasses.

For this type of alkali-free glass, a batch decomposition occursgenerally at 1,300-1,500° C. Accordingly, fining and homogenization arecarried out at 1,500° C. or higher temperatures. Under such conditions,As₂O₃ has been widely used as a fining agent which can release a fininggas over a wide temperature range (approximate range of 1,300-1,700°C.).

However, As₂O₃ is very toxic in nature and possibly causes environmentalpollution during glass manufacturing processes or waste glasstreatments. For this reason, its use is being increasingly limited.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an alkali-free glasssubstrate rendered free of bubbles resulting in display defects withoutusing As₂O₃ as a fining agent.

After conduction of various experiments, the inventors of thisapplication have found that the above object can be achieved by usingSb₂O₃ or SnO₂ as a fining agent and controlling a water content in theglass at at least a specific level.

That is, the alkali-free glass substrate of the present invention ischaracterized as comprising a glass which has a β-OH value of at least0.485/mm and contains SnO₂ and/or Sb₂O₃.

The preferred β-OH value is 0.5/mm or larger.

The alkali-free glass substrate preferably comprises a glass containing,by mass percent, 50-70% SiO₂, 10-25% Al₂O₃, 5-20% B₂O₃, 0-10% MgO, 0-15%CaO, 0-10% BaO, 0-10% SrO and 0-5% ZnO.

An As₂O₃ content is preferably 0.4% or less by mass. The SnO₂ content ispreferably 0.05-1% by mass. The Sb₂O₃ content is preferably 0.05-3% bymass. A Cl₂ content is preferably 0.1% or less by mass.

The glass substrate preferably has an area of at least 0.5 m².

The alkali-free glass substrate of the present invention is suitable foruse as a transparent glass substrate of a liquid crystal display.

Because the alkali-free glass substrate of the present inventioncomprises a glass rendered free of bubbles resulting in display defectswithout use of As₂O₃, it can be suitably used as a transparent glasssubstrate for displays. Particularly, this alkali-free glass substratebecomes very advantageous when used in a large size, because such useincreases a percentage of non-defectives.

Also, the increased β-OH value enables reduction of the amount of B₂O₃and improves chemical durability.

BEST MODE FOR CARRYING OUT THE INVENTION

Japanese Patent Kohyo No. 2001-500098 discloses that if the β-OH valueis maintained below 0.5/mm, preferably below 0.45/mm, formation ofbubbles at a platinum interface can be prevented. However, clearing ofbubbles produced during an initial stage of melting (e.g., during abatch decomposition), what is called, a fineness improvement is notconsidered at all in Japanese Patent Kohyo No. 2001-500098. The β-OHvalue disclosed in Japanese Patent Kohyo No. 2001-500098 indicates awater content level obtained under normal melting conditions. In otherwords, the disclosed β-OH level is comparable to those for conventionalalkali-free glasses.

Surprisingly, it has been clearly shown that the glass is better finedif the β-OH value is increased to at least a specific value. In thisinvention, the water content in the glass is adjusted to a high levelwhich is difficult to be attained under normal melting conditions. Thiscontemplates to compensate for the deficiency in fining ability thatresults from the use of SnO₂ or Sb₂O₃ as an alternative fining agent ofAs₂O₃. The water present in the glass acts to lower viscosity of theglass and, if contained in a large amount, facilitates melting andfining of the glass. Also, the water itself serves as one fining gas todiffuse into bubbles, increase bubble sizes and allow them to float up.

Specifically, Sb₂O₃ releases a fining gas in a lower temperature rangecompared to As₂O₃, and accordingly use of Sb₂O₃ in a high temperaturerange (e.g., homogenization melting temperature range) brings a slightshortage of the fining gas. In order to counter such a shortage, thepresent invention contemplates to allow the glass to contain a largeamount of water. The water, if present in a large amount, serves as afining gas to diffuse into bubbles in the temperature range, therebycompensating for the shortage in volume of the fining gas. On the otherhand, SnO₂ releases a fining gas in a higher temperature range comparedto As₂O₃, and accordingly use of SnO₂ in a low temperature range (e.g.,a batch decomposition temperature range) brings a slight shortage of thefining gas. However, the water, if present in a large amount, lowersviscosity of the glass and facilitates melting thereof in the lowtemperature range. As a result, the glass is better fined.

The water content in the glass, as indicated by the β-OH value, is atleast 0.485/mm, preferably at least 0.5/mm, most preferably at least0.51/mm. As the water content in the glass increases, the glassviscosity decreases, while the amount of the water diffusing intobubbles increases. As a result, the glass is better fined.

The higher the β-OH value, the better the glass is melted. On the otherhand, as the β-OH value increases, the strain point tends to decrease.For such reasons, it is desirable that the β-OH value does not exceed0.65/mm, particularly 0.6/mm.

The β-OH value, indicative of the water content in the glass, can becalculated from the following equation:β-OH=(1/X)LOG₁₀(T ₁ /T ₂)where,

-   -   X=glass thickness (mm);    -   T₁=transmittance (%) at the reference wavelength 3,846 cm⁻¹        (2,600 nm); and    -   T₂=minimum transmittance (%) at and near the hydroxyl absorption        wavelength 3,600 cm⁻¹ (2,800 nm).

The glass substrate of the present invention preferably comprises analuminosilicate glass. More specifically, it desirably comprises analkali-free glass having a basic composition containing, by masspercentage, 50-70% SiO₂, 10-25% Al₂O₃, 5-20% B₂O₃, 0-10% MgO, 0-15% CaO,0-10% BaO, 0-10% SrO and 0-5% ZnO. By “alkali-free”, as used in thepresent invention, it is meant that an alkaline metal oxide (Li₂O, Na₂O,K₂O) content is up to 0.2% by mass.

The reasons why the composition of the alkali-free glass substrateshould be brought within the above-specified range are given below.

SiO₂ is a network component of the glass. Its content is 50-70%,preferably 55-70%, more preferably 55-65%. If the SiO₂ content is below50%, the chemical resistance of the glass is lowered. Also, its strainpoint is lowered so that the heat resistance of the glass deteriorates.The SiO₂ content of exceeding 70% increases the high-temperatureviscosity of the glass so that its melting ability is lowered, as wellas increasing the tendency of cristobalite to precipitate viadevitrification.

Al₂O₃ is a component which improves heat resistance and devitrificationtendency of the glass. Its content is 10-25%, preferably 10-20%, morepreferably 13-18%. If the Al₂O₃ content is less than 10%, adevitrification temperature is markedly elevated to result in theincreased occurrence of devitrification in the glass. If the contentexceeds 25%, the acid resistance, particularly the resistance tobuffered hydrofluoric acid, is lowered to result in the increasedoccurrence of haze at a surface of the glass substrate.

B₂O₃ is a component which acts as a flux to lower the viscosity andthereby facilitate melting of the glass. Its content is 5-20%,preferably 5-15%, more preferably 8.5-12%. If the B₂O₃ content is lessthan 5%, its effect as a flux becomes insufficient. On the other hand,the higher B₂O₃ content tends to lower the acid resistance. Particularlywhen the B₂O₃ content is greater than 15%, the resistance of the glassdecreases against hydrochloric acid and its strain point drops so thatthe heat resistance is lowered.

As described above, B₂O₃ is a component which affects the acidresistance of the glass. The reduction of its content improves the aciddurability of the glass. A metal film or an ITO film is formed on asurface of a transparent glass substrate for use in liquid crystaldisplays. Since such a film is patterned by acid etching, the glass musthave a high degree of acid resistance. It is accordingly desirable toreduce the B₂O₃ content in the glass. Since boron (B) is a chemicalsubstance designated in the PRTR Act, the reduction of the B₂O₃ contentis also desirable from an environmental point of view. However, simplereduction of the B₂O₃ content may in turn raise other problems,including deterioration of the melting ability and increase of bubbles.In the present invention, the β-OH value of the glass is maintained at ahigh level. This restrains elevation of glass viscosity that occurs whenthe B₂O₃ content is reduced. It is believed that as the β-OH valueincreases, the glass viscosity decreases to thereby facilitate reductionof the B₂O₃ content.

MgO is a component which lowers the high-temperature viscosity of theglass without a drop of the strain point and thereby facilitates meltingof the glass. Its content is 0-10%, preferably 0-7%, more preferably0-3.5%. If the MgO content is greater than 10%, the glass exhibits amarked reduction in resistance to buffered hydrofluoric acid.

CaO functions in a manner similar to MgO. Its content is 0-15%,preferably 5-10%. If the CaO content is greater than 15%, the glassexhibits a marked reduction in resistance to buffered hydrofluoric acid.

BaO is a component which improves not only the chemical durability butalso devitrification tendency of the glass. Its content is 0-10%,preferably 0-7%. If the BaO content is greater than 10%, a strain pointdrops to deteriorate the heat resistance of the glass.

SrO affects in a manner similar to BaO. Its content is 0-10%, preferably0-7%, more preferably 0.1-7%. If the SrO content exceeds 10%, theoccurrence of devitrification undesirably increases.

For portable devices such as portable telephones and notebook computers,the lightweight device designs have been required to increase handinessduring carriage. In order to achieve such lightweight designs, a glasssubstrate for use therein must be reduced in density. Preferably, thistype of glass substrate exhibits a low thermal expansion coefficient asapproximate to that (about 30-33×10⁻⁷/° C.) of a thin-film transistor(TFT) material, specifically a thermal expansion coefficient of28-40×10⁻⁷/° C., because a large difference in thermal expansioncoefficient between the glass substrate and the TFT material results inthe occurrence of warp. BaO and SrO are the glass components which alsoaffect the density and thermal expansion coefficient of the glass. Inorder to obtain a low-density and low-expansion glass, their totalcontent must be controlled not to exceed 15%, preferably 10%.

ZnO is a component which improves resistance to buffered hydrofluoricacid as well as divitrification resistance. Its content is 0-10%,preferably 0-5%. If the ZnO content exceeds 10%, the occurrence ofdevitrification of the glass increases. Also, a strain point drops toresult in the failure to obtain sufficient heat resistance.

The glass contains at least one of SnO₂ and Sb₂O₃ as an essential finingcomponent. The SnO₂ content is between 0.05 and 1%, with 0.05-0.5% beingparticularly preferred. The Sb₂O₃ content is 0.05-3%, with 0.05-2% beingparticularly preferred. SnO₂ shows a tendency to precipitate in theglass when its content exceeds 1%. If the Sb₂O₃ content exceeds 3%, thedensity of the glass increases. Also, the strain point of the glass isdropped to deteriorate its heat resistance. If the SnO₂ or Sb₂O₃ contentis below 0.05%, it becomes difficult to obtain a sufficient finingeffect.

From an environmental point of view, the As₂O₃ content should bemaintained at as low a level as possible, preferably within 0.4%, morepreferably within 0.1%, further preferably within 0.05%.

Besides the aforementioned components, the glass in the presentinvention may further contain ZrO₂, TiO₂, Fe₂O₃, P₂O₅, Y₂O₃, Nb₂O₃ andLa₂O₃ in the total amount of up to 5%, for example. CeO₂, MnO₂, WO₃,Ta₂O₅, Nb₂O₅, SO₃, chlorides and fluorides can also be used as a finingagent. However, the present invention finds chlorides as being inferiorto the others, because they cause a marked reduction of the watercontent in the glass. The use amount of a chloride is limited to 0.1% orbelow, particularly to 0.04% or below, in terms of a Cl₂ contentcalculated from a chlorine component remaining in the glass. It would beadvisable to exclude the chloride, if possible.

A method of producing the alkali-free glass substrate of the presentinvention is below described.

First, a glass batch is prepared such that a glass having a desiredcomposition results.

Then, the prepared glass batch is melted.

Thereafter, the molten glass is formed into a desired shape to obtain aglass substrate. For display applications, the molten glass may beformed into a thin sheet using an overflow downdraw method, a slotdowndraw method, or a float method. Use of the overflow downdraw methodis particularly preferred because it results in obtaining a glass sheethaving a very superior surface quality.

In the above production process, the β-OH value of an alkali-freealuminosilicate glass having the above composition does not exceed0.485/mm, particularly 0.5/mm, unless a proper measure is positivelydevised to increase a water content.

For this reason, in the production of the alkali-free glass of thepresent invention, various measures should be devised to increase thewater content. For example, one or more of the following measures may beadopted selectively: (1) a high water-content batch material (e.g., ahydroxide material) is selected; (2) water is added to the batch; (3) acomponent (e.g., chlorine) which lowers the water content in the glass,if used, is reduced in amount or excluded; (4) oxygen combustion iscarried out during melting of the glass, or a water vapor is introduceddirectly into a melting furnace to thereby increase the water content inan atmosphere inside the furnace; (5) a water vapor bubbling is carriedout through the molten glass; and (6) a large-scale melting furnace isemployed, or a flow rate of the molten glass is slowed down to therebyallow the molten glass to stay for an extended period in a meltingfurnace under the atmosphere controlled at a high water content.

The thus-produced, alkali-free glass substrate of the present inventionis characterized as including a very small number of bubbles. Ingeneral, a percentage of non-defectives drops dramatically owing tobubbles in glass if a substrate is rendered larger in size. Hence, thereduction in number of bubbles is very advantageous for a large-scalesubstrate such as having an area of 0.5 m² or larger (specifically 630mm×830 mm or larger), particularly 1.0 m² or larger (specifically 950mm×1,150 mm or larger), further 2.5 m² or larger (specifically 1,450mm×1,750 mm or larger). The suitable number of bubbles is within therange that does not exceed 0.2/kg, preferably 0.1/kg, more preferably0.05/kg.

DESCRIPTION OF PREFERRED EXAMPLES

The following example illustrates the alkali-free glass substrate of thepresent invention. TABLE 1 Sample No. 1 2 3 4 5 Glass Composition SiO₂59.5 59.4 59.4 59.6 59.4 Al₂O₃ 15.3 15.2 15.4 14.8 15.3 B₂O₃ 9.9 9.6 9.99.8 9.8 MgO 0.02 0.02 0.02 0.01 0.01 CaO 5.40 5.36 5.50 5.44 5.45 SrO6.10 6.09 6.07 6.07 6.01 BaO 2.20 2.19 2.19 2.21 2.19 ZnO 0.40 0.42 0.400.48 0.48 ZrO₂ 0.10 0.15 0.10 0.05 0.08 As₂O₃ 0.02 0.33 0.70 0.06 0.03Sb₂O₃ 0.92 0.77 0.36 0.90 0.94 SnO₂ 0.18 0.13 0.01 0.18 0.06 Cl₂ <0.010.01 0.05 0.13 0.23 β-OH (/mm) 0.540 0.512 0.419 0.292 0.241 Number ofBubbles 0.10 0.08 0.03 0.85 35.6 (/kg) Density (g/cm²) Un- 2.502 Un- Un-Un- meas- meas- meas- meas- ured ured ured ured Expansion Un- 37.3 Un-Un- Un- Coefficient meas- meas- meas- meas- (×10⁻⁷/° C.) ured ured uredured Strain Point (° C.) Un- 652 Un- Un- Un- Annealing Point (° C.)meas- 708 meas- meas- meas- Softening Point (° C.) ured 955 ured uredured 10⁴ (° C.) 1291 10³ (° C.) 1459 10^(2.5) (° C.) 1570

Each sample shown in the Table was fabricated in accordance with thefollowing procedure.

First, a glass batch having the composition specified in the Table wasprepared, mixed and then melted in a continuous melting furnace at amaximum temperature of 1,650° C. The molten glass was formed into asheet by an overflow downdraw method and then cut to obtain analkali-free substrate sample having a size of 1,000×1,200×0.7 mm forvarious evaluations. In the fabrication of the sample No. 1, a watercontent in an atmosphere inside the melting furnace was increased toincrease a water content of the resulting glass. In the fabrication ofthe sample No. 2, a high water-content batch material was selected toincrease a water content of the resulting glass The β-OH value of eachglass was determined by measuring its light transmittance using FT-IRand making a calculation using the following equation:β-OH value=(1/X)LOG₁₀(T ₁ /T ₂)where,

-   -   X=glass thickness (mm);    -   T₁=transmittance (%) at the reference wavelength 3,846 cm⁻¹; and    -   T₂=minimum transmittance (%) at and near the hydroxyl absorption        wavelength 3,600 cm⁻¹.

The fineness was evaluated by counting the number of 100 μm and largersize bubbles present in the glass and reducing the result to the numberof bubbles per kg of the glass. The density was determined by awell-known Archimedes' method. The thermal expansion coefficient wasgiven in terms of a mean thermal expansion coefficient measured over the30-380 C temperature range using a dilatometer. The strain point andannealing point were measured according to the method specified in ASTMC336-71. The softening point was measured according to the methodspecified in ASTM C338-73. A platinum ball pull-up method was utilizedto measure temperatures which correspond to viscosities at 10⁴, 10³ and10^(2.5).

Utility in Industry

The alkali-free glass substrate of the present invention has applicationon liquid crystal displays and others. For example, it can be used as aglass substrate for flat displays such as electroluminescent displays;as a cover glass for image sensors such as charge-coupled devices,equimultiple solid-state image pickup proximity sensors and CMOS imagesensors; and as a glass substrate for hard disks and filters.

1. An alkali-free glass substrate characterized as comprising a glasshaving a β-OH value of at least 0.485/mm and containing SnO₂ and/orSb₂O₃.
 2. The alkali-free glass substrate as recited in claim 1,characterized in that said B-OH value is at least 0.5/mm.
 3. Thealkali-free glass substrate as recited in claim 1, characterized in thatsaid glass contains, by mass percent, 50-70% of SiO₂, 10-25% of Al₂O₃,5-20% of B₂O₃, 0-10% of MgO, 0-15% of CaO, 0-10% of BaO, 0-10% of SrOand 0-5% of ZnO.
 4. The alkali-free glass substrate as recited in claim1, characterized as having an As₂O₃ content of 0.4% or less by mass. 5.The alkali-free glass substrate as recited in claim 1, characterized ashaving the SnO₂ content of 0.05-1% by mass.
 6. The alkali-free glasssubstrate as recited in claim 1, characterized as having the Sb₂O₃content of 0.05-3% by mass.
 7. The alkali-free glass substrate asrecited in claim 1, characterized as having a Cl₂ content of 0.1% orless by mass.
 8. The alkali-free glass substrate as recited in claim 1,characterized in that said substrate has an area of at least 0.5 m². 9.The alkali-free glass substrate as recited in claim 1, characterized inthat said glass substrate is used as a transparent glass substrate for aliquid crystal display.